Lithium iron phosphate LiFePO4 used as a cathode material in a secondary battery such as a metal lithium battery, lithium ion battery or lithium polymer battery is subjected to electrode oxidation/reduction accompanied by doping/undoping of lithium during the process of charging and discharging. Lithium iron phosphate LiFePO4 is expected as a highly potential cathode material in the next generation because it has a considerably large theoretical capacity (170 mAh/g) and can create a relatively high electromotive force (about 3.4 to 3.5 V at Li/Li+ anode) and because it is considered to be produced at low cost since it can be produced from iron and phosphorus, which are abundant resources. An LiFePO4 cathode system having an olivine-type crystal structure, unlike a number of other currently-available cathode systems such as a lithium cobaltate [LiCoO2] cathode system, is in a two-phase equilibrium state in which only a reduced form (discharged state) LiFe(II)PO4 as a first phase into which Li has been fully inserted and an oxidized form (charged state) Fe(III)PO4 as a second phase from which Li has been completely extracted exist [that is, no intermediate phase, such as Li0.5(Fe2+0.5Fe3+0.5)PO4, is not formed] all through the electrode oxidation/reduction process. As a result, the cathode system has an interesting property that the charge/discharge voltage is always kept constant and thus its charge/discharge state is easy to control. However, both the oxidized form (discharged state) LiFe(II)PO4 and Li-extracted oxidized reduced form (charged state) Fe(III)PO4 have extremely low conductivities, and Li+ ions cannot move quickly in the cathode material (the two features are assumed to be associated with each other as described later). Thus, even when a secondary battery is fabricated using Li or the like in the anode, only a small effective capacity, bad rate characteristics and bad cycle characteristics can be obtained. As a method for enhancing the surface conductivity of a cathode material, there has been disclosed a process for depositing carbon on the surfaces of particles of a complex oxide (including an oxo acid salt such as sulfate, phosphate or silicate) represented by the chemical formula AaMmZzOoNnFf (wherein A represents an alkali metal atom, M represents Fe, Mn, V Ti, Mo, Nb, W or other transition metal atom, and Z represents S, Se, P, As, Si, Ge, B, Sn or other non-metal atom). When the composite material is used in the electrode system of a battery, the electric field around interfaces of the complex oxide particles, a current collector (conductivity-imparting) material and an electrolyte can be uniform and stable and the efficiency can be improved in the course of electrode oxidation/reduction (see Document 1). To deposit carbon on the surfaces of the complex oxide particles, an organic substance (polymer, monomer, or low-molecular weight compound) from which carbon is formed by pyrolysis or carbon monoxide is added to the complex oxide and pyrolyzed (a composite material of the complex oxide and surface covering carbon can be obtained by thermal reaction of the organic substance and the ingredients of the complex oxide under reducing conditions). According to Document 1, an improvement in the surface conductivity of the complex oxide particles can be realized by the method, and high electrode performance such as high discharge capacity can be achieved when Li polymer battery is produced using a composite material prepared by depositing carbon on the surfaces of particles of a cathode material such as LiFePO4. There has been also disclosed a method for producing a cathode active material comprising the steps of mixing and milling ingredients of a compound represented by the general formula LixFePO4 (wherein 0<x≦1), and calcining the mixture in an atmosphere with an oxygen content of 1012 ppm (by volume) or lower, wherein a non-crystalline carbon material such as acetylene black is added at any point in the processing (see Document 2).
The above techniques are applied to improve the cathode performance, both based on the low conductivity of a phosphate cathode material such as LiFePO4 and the slow movement of Li ions in the cathode material. Basically, the techniques try to avoid these difficulties by depositing a conductive substance such as carbon on the surface of the cathode material or adding a conductive substance to the cathode material and reducing the particle size of the cathode material as much as possible to limit the ion diffusion distance.
Attempts have been made to improve the cathode performance by enhancing the conductivity of a LiFePO4 cathode material by replacing some of Li or Fe of the cathode material with different metal elements, or compositing or doping some of Li or Fe of the cathode material with different metal elements (see Documents 3 and 4, for example). Document 3 discloses that when Al, Ca, Ni or Mg is introduced into the LiFePO4 cathode material, its capacity can be improved. It is, for example, reported that a metal lithium battery using the LiFePO4 cathode material free of the above elements exhibited a discharge capacity of 117 mAh/g in the first cycle and the rapid discharge capacity decreases with the progress of the cycle whereas a battery using a LiMg0.05Fe0.95PO4 cathode material obtained by replacing some of Fe of the LiFePO4 cathode material with Mg exhibited a discharge capacity of about 120 to 125 mAh/g and less deterioration with the progress of the cycle (although no objective evidence which indicates that Fe is replaced with Mg in the cathode material is shown).
Document 4 discloses that cathode materials into which the elements Mg, Al, Ti, Zr, Nb and W are doped, respectively, are produced by adding compounds containing Mg2+, Al3+, Ti4+, Zr4+, Nb5+ and W6+ (Mg is in the form of an oxalate, Nb is in the form of a metal phenoxide, and the others are in the form of metal alkoxides) respectively to the ingredients of a LiFePO4 cathode material and calcining the mixtures. It is assumed in the document that the materials have some of their Li replaced with each of the elements and exist in the form of Li1-xMxFePO4. It is also reported that the metal ion-doped cathode materials had conductivities in the order of 10−1 to 10−2 S/cm, which is about 108 times greater than that of the non-doped cathode material, at room temperature, and metal lithium batteries using the metal ion doped cathode materials with such high conductivities had excellent rate characteristics and a long cycle life. According to Document 4, one of the metal lithium batteries exhibited a discharge capacity slightly greater than 140 mAh/g at a low charge/discharge rate of C/10 (although the discharge capacity is described as about 150 mAh/g in the document, it is close to 140 mAh/g as long as seen in an accompanying drawing), and was able to be stably charged and discharged cyclically at very high rates of 21.5 C and 40 C, exhibiting reduced discharge capacities of slightly lower than 70 mAh/g and about 30 mAh/g, respectively (C/n is the rate of charging or discharging a battery under constant current, wherein n is the number of hours in which the battery is completely charged or discharged. There is no description in the document about the dopant element from which the charge/discharge data were derived and its content in the cathode material.).
It is assumed in Document 4 that since a small amount (less than 1 mol %, in terms of element ratio, based on iron) of the polyvalent ions enter the sites of Li+ ions in the crystal structure of the reduced form LiFe(II)PO4 of the cathode material and its Li-extracted oxidized form Fe(III)PO4, a small amount of Fe3+ and Fe2+ are generated in the reduced phase and the oxidized phase, respectively, to create an oxidized state in which Fe2+ and Fe3+ coexist, and, consequently, P-type semiconductivity and N-type semiconductivity appear in the reduced phase and the oxidized phase, respectively, and provides the improvement in the conductivity. It is also reported that when the LiFePO4 cathode material was calcined together with any of the compounds containing the above bivalent to hexavalent ions, the conductivity of the cathode material was also improved (since the transition metal elements Ti, Zr, Nb and W can be in the form of stable positive ions with different valences, the valences of the positive ions in the obtained cathode materials may be different from those of the compounds added for doping).    Document 1: JP-A 2001-15111    Document 2: JP-A 2002-110163    Document 3: “Research for the Future Program, Tatsumisago Research Project: Preparation and Application of Newly Designed Solid Electrolytes (Japan Society for the Promotion of Science: Research Project No. JSPS-RFTF96PO010) [http://chem.sci.hyogo-u.ac.jp/ndse/index.html] (updated on Jun. 21, 2000)    Document 4: Nature Materials Vol. 1, pp. 123 to 128 (October, 2002)
The methods disclosed in Documents 3 and 4, however, cannot provide satisfactory results at the moment. The charge/discharge capacity achieved by the former method is 120 to 125 mAh/g at best. In addition, although the adaptability of the latter to high-rate charging/discharging is remarkable, only a charge/discharge capacity much smaller than the theoretical capacity of the cathode material 170 mAh/g can be obtained (slightly higher than 140 mAh/g) even at a low rate of C/10 in spite of the fact that the conductivity of the LiFePO4 cathode material is improved. Further, the rise/fall of voltage in the final stage of charge or discharge under constant current in the battery capacity-voltage characteristic curve is not very steep in spite of the high-rate characteristics. According to the data shown in Document 4, the voltage has a gentle rise/fall from points about 80% of the depths of charge and discharge at a rate of C/10. In a battery having a small internal resistance and high-rate characteristics, however, the rise/fall of voltage should be as steep as 90 degrees. The facts suggest the possibility that the type of the composited or doped element and the compositing or doping method is not fully appropriate.